Production of flat high strength steel sheets



April 3, 1962 R. w. HOLMAN 3,028,458

PRODUCTION OF FLAT HIGH STRENGTH STEEL SHEETS Filed July 31, 1959INVENTOR ROBERT W HOLMA/V United States Patent M 3,028,468 PRODUCTION OFFLAT HIGH STRENGTH STEEL SHEETS Robert W. Holman, Birmingham, Ala.,assignor to United States Steel Corporation, a corporation of New JerseyFiled July 31, 1959, Ser. No. 830,947 2 Claims. (Cl. 219--7.5)

This application relates to improvements in the production of highstrength steel sheets and more particularly to the production of flathigh strength steel sheets.

It has heretofore been proposed to produce steel sheets having highstrength by forming the sheets of hardenable steels and then heattreating, i.e. quenching from above the critical temperature of thesteel and tempering to produce the desired combination of physicalproperties. However, the heat treatment, particularly in sheet gaugesresults in considerable warpage and distortion. Attempts to flatten thesheets by conventional stretcher levelling and the like have not beensuccessful because of the high incidence of breakage even at elongationstoo small to produce the desired flatness.

It is accordingly an object of this invention to provide an improvedmethod of flattening high strength steel sheets.

It is a further object of this invention to provide an improved methodof stretcher levelling high yield strength notch sensitive steel sheets.

The foregoing and further objects will be apparent from the followingspecification when read in conjunction with the accompanying drawing,wherein:

FIGURE 1 is a plan view; and

FIGURE 2 is a side elevation.

Referring more particularly to the drawing, the numeral 2 indicates asuitable conveyor table for a sheet 4. The sheet 4 may be advanced byfeed rolls 6 driven by suitable motors and gear trains 8. Adjustableside guides 10 are adapted to properly position the sheet 4 with respectto high frequency induction heaters 12 disposed on opposite sides of theconveyor table. The motors and feed rolls are adapted to advance thesheet past the induction heaters 12 at the desired rate to heat theedges of the sheet to slightly below, i.e. within about 100, of thecritical temperature of the steel thereof.

A conventional stretcher leveler for flattening the sheets after edgetreatment is shown in FIGURE 2. This comprises a pair of gripper members20, one of which is fixedly mounted and the other movably mounted. Themovably mounted member may be actuated by a hydraulic cylinder 22 tostretch a sheet 4 held by the gripper members.

In practicing my invention, a hardened and tempered sheet 4 which may becomposed Of a ha de b e W6 containing about 3,028,468 Patented Apr. 3,1962 Percent .25 carbon 12.0 chromium 1.0 molybdenum .25 vanadium Thesheet is placed on the conveyor 2 and drawn between the inductionheaters. The induction heaters and the speed of advance of the strip areregulated to heat the sheet edges to about 1200 F., i.e. slightly belowthe critical temperature to a transverse depth of about 1 to 5 times thestrip thicknesses in a very short time. By heating sufficiently rapidly,the heat imparted to the edges can flow inwardly of the stripsufficiently rapidly to quench the edges.

Thereafter the edge treated strip is placed in the stretcher levellerand stretched to a suflicient elongation to flatten it. This eliminatesbreakage due to edge sensitivity.

While I have shown and described a specific embodiment of my invention,it will be understood that this embodiment is merely for the purpose ofillustration and description and that various other forms may be devisedwithin the scope of my invention, as defined in the appended claims.

I claim: s

1. In the method of stretcher flattening heat-treatment hardened steelsheets characterized by freedom of edge cracking after stretcherflattening the improvement comprising quickly heating the edge portionsonly of said sheets to slightly below the lower critical temperature ofthe steel at a rate sufliciently rapid that the heated edge portionswill be quenched by inflow of heat therefrom into the body of the sheetprior to stretcher flattening the sheet and then stretching said sheetsto flatten them.

2. In the method of stretcher flattening heat-treatment hardened steelsheets characterized by freedom from edge cracking after stretcherflattening the improvement comprising quickly heating the edge portionsonly of said sheets to a depth of about 1 t0 5 times the strip thicknessto a temperature slightly below the lower critical temperature of thesteel at a rate sufficiently rapid that the heated edge portions will bequenched by inflow of heat therefrom into the body of the sheet prior tostretcher flattening the sheet and then stretching said sheets toflatten them.

References Cited in the file of this patent UNITED STATES PATENTS2,448,062 Stoltz Aug. 31, 1948 2,477,411 King July 26, 1949 2,507,817Ropp May 16, 1950 2,716,688 Lackner Aug. 30, 1955 2,873,344 Kooks Feb.10, 1959

